INP measures responsiveness: when someone taps, clicks, or types, how long until the page visibly reacts. It replaced the older First Input Delay metric because it captures responsiveness across the whole visit, not just the first interaction. Google considers 200 milliseconds or less to be good.
Heavy JavaScript is the usual culprit. Breaking up long tasks, deferring non-essential scripts, and doing less work on the main thread all help.
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